1. Field of the Invention
The present invention concerns a method to design a gradient coil, a method to manufacture a gradient coil, a gradient coil, a magnetic resonance apparatus and a combined PET-MR system.
2. Description of the Prior Art
Known medical imaging techniques include nuclear medicine techniques that primarily image functional processes in an examination subject, as well as techniques that primarily image anatomy (for example magnetic resonance techniques and computed tomography techniques).
The PET technique (PET: positron emission tomography) is among the nuclear medicine imaging techniques. PET generates images of living organisms in that it shows the distribution of a previously administered, weakly radioactive marked substance (radiopharmaceutical) in the organism that has accumulated in the organism such that biochemical and physiological processes can be imaged.
Radionuclides that emit positrons upon decay are hereby suitable as radiopharmaceuticals. The positrons interact with an electron at a short distance (approximately 2-3 mm) and this leads to what is known as annihilation. Both particles (positron and electron) are thereby annihilated, and two high-energy photons (gamma radiation) arise with 511 keV each. They depart at an angle of approximately 180° from one another. The line that is thereby formed is also designated as a “line of response” (LOR). The two photons (annihilation radiation) are measured, for example at a detector ring where they simultaneously impact at two points. Confirmation of the positron emission and an estimation of the location of the annihilation are possible due to the coincidence of the two measurement results.
The magnetic resonance technique (in the following the abbreviation MR stands for magnetic resonance) is a known technique with which images of the inside of an examination subject can be generated. Expressed simply, for this the examination subject is positioned in a comparably strong, static, homogeneous basic magnetic field (field strengths from 0.2 Tesla to 7 Tesla and more) so that its nuclear spins orient along the basic magnetic field. To trigger nuclear magnetic resonances, radio-frequency excitation pulses are radiated into the examination subject, the triggered nuclear magnetic resonances are measured and MR images are reconstructed based on these. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field in three spatial directions. The acquired measurement data are digitized and stored in a k-space matrix as complex numerical values. By means of a multidimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated with values. This technique allows an excellent depiction (in particular of soft tissues) with selectable contrasts.
In order to be able to utilize the advantages of both techniques synergistically, efforts have been made to combine MR and PET systems. One example of a combined PET/MR system with a PET detector suitable for MR is described in United States Patent Application Publication No. US 2007/0102641.
Furthermore, there exists a need for improved components for combined PET-MR apparatuses as well as for magnetic resonance apparatuses.